Telephone system



A ril 18, 19 "R. s. RICHARDSON TELEPHONE SYSTEM Filed June "8, 1931 1 s Sheets-Sheet 1 Ap 1933. R. G. RICHARDSON TELEPHONE SYSTEM Filed June 8, 1931 3 Sheets-Sheet 2 Izmir- Rn dni E. Ric'hardgun Ap i 8, 1933- R. e. RICHARVDSON TELEPHONE SYSTEM 3 Sheets-Sheet 3 Filed June 8, 1931 Inuen ur H 1: ans E: Richards on was Apr. is, 1.933

vm.'r1: *s'rAT PAT "om m o. nrcxumson, or cmcascnmmoza meson, BY minnows. roassocnup nxc'rarc maoauonms, me, or cmcaoo, more. a comm Eamon SISTER,

ya uuaund an aim. lei-la! Io. mm.

The present invention relates to telephone systems in general, but is concerned more particularly with telephone systems employing automatic switches .to set up desired connections; and the main object is the'production of new and improved circuit arrangements in automatic switches for efiecting the' necessary. testing operations over one of the talking conductors and forcontrolling the release of, the automatic switches over one of the talking conductors, thereby. eliminating the test. and holding conductor ordinarily provided between successive automatic switches in an established connection and eliminating also the corresponding -wipare or brushes on the automatic switches themselves.

General description Q- It is well known that one or more selec-. tors and a connector are successively operated inan automatic telephone system to extend. the two conductors of a calling line into telephone connections ductors of a called line, and that a third conductor is ordinarily employed inthe trunk extending from one automatic switch to the ing conductors are extended through the wipers thereof; to, the next switch in the train and are disconnected from any relay apparatus of the selector, making it neces-' sary to control the releasing of the'selector over the third conductor. This third con- :ductor in. an established connection i'slnot used. for talking purposes, and mayfthere fore be eliminated if satisfactory arrange ments are developed to enable the. necessary testing and holding, or. 're lease function to be'carried out in some otherma er. as, for,

example, over one of the talking conductors. It is realized, of course, that numerous attempts-have been made to solve the'problem heretofore, but so far as applicant is aware," these. attempts have been: unsuccess-' ful, chiefly because the apparatus involved with the-two con .su t is accomp has,'for .the most part, resulted in an impaired talking connection because of the serice or multiple holding connections, and have causedthe established talking connection to be noisy because of the disturbances resulting from tests being performed by sub sequently operated switches'ofi portions of the established connections. Accordingly the arrangement disclosed herein is intended to avoid these difliculties by employing vacuum tubes tomake theneoessary tests byway v of their, grid elements and-tb control the releasing operations by having their grid elements connected to the talking circuits. It will be understood, of course, that the" 'd element of a vacuum tubedoes, not or arily consume an appreciable amount of current from the circuit to which it is connected,

for which reason it is well adaptedlto the purpose above set to It is a further bbject of the invention to eliminate the third conductor from the multiple of a subscribers line, which third conductor is ordinarily used for testing purposes.

and to energi'zethe cut-ofi relay. As will be ex lainedmore in .detail hereinafter, this rear the cutofi relay to be opera over one o the talking conductors, and by the apparatus that tests subscribers lines so that the of such lines is' carried out over one of the talkin conductors bZeconnecting the grid element 0 a vacuum tu thereto.

An additional object is to. provide suitable circuit arrangements in the automatic switches so that a vacuum tube may be used :to respond to the controlling impulses received from the calling line-and repeat them to the o irating magnets of the switch, after which t fe'vacuumjtube is shifted into connection with one of the wipers of the to enable it-to be 'used to test trunk lines, as'

inthe case of the selector, and 'to test the called line, as'i'n the case of a connector.

'Itis a further object to provide suitable, circuit arrangements, whereby an established talking connection is not materially unbalandthereby rendered noisy because the cut-.ofi relays of the lines involved are connected to one of the conductors of the, estabtion of the talkin lished talking connection. In one embodiment, the talking conductors are reversed between the incoming conductors of the connector and the wipers thereof, so that the cut-off rela of the called line is on the opposite side 0 the established connection from the cut-off relay of the calling line, thereby making the two sides of the established talk-1 ing circuit symmetrical except for the slight di erence in location of the two cut-ofl relays and the impedance of the intervening porcircuit.

In another mo ification, arrangements 'are provided for placing a balancing coil on the other talking conductor near the calling line to balance the cut-ofl' relay of the calling line and for placing a balancing-coil near the called line and on the opposite conductor from the one used to operate the cut-off relay of the called line.

Other objects and features are concerned with the provision of circuit arrangements evolved to carry out the arrangements hereinbefore outlined and to produce a complete working automatic telephone system wherein the third conductor is omitted in the line and trunk'multiples.

Desmiptz'on of drawings Referring now to the accompanying drawings comprising Figs. 1-3, they sh w by means of the usual circuit diagrams g sufficient amount of the equipment in a, telephone system embodying the features of the mvention to enable the invention to be understood. Figs. 1 and 2 show suflicient automatic equipment for setting up a connection from the line of substation A1, Fig. 1, to the line of substation A2, Fig. 2, which equipment includes a line. finder LFl and ;a selector S1, Fig. 1, and a connector C1, Fig. 2. Fig. 3 shows a modified line finder, selector, and connector which maybe used in setting up a connection from the line of substation A3 to the line of substation A4.

Mechanically, the 'finder, selector, and connector switches shown in the drawings are of the usual Strowger type, having their bank contacts arranged in ten levels, with ten sets of contacts in a level. The selector S1, Fig. 1, is one of a group of selectors having access to ten groups of connectors, of which the connector C1, Fig. 2, is one of the connectors in the cup accessible by way of the third level of ank contacts of the selectors. .b'ince each connector group has access to amaximum of one hundred lines, the ultimate capacity of the system shown in the drawings is one thousand lines. It will be understood, of course, that this capacity can be increased to any desired extent by including as many selector groups as necessary, in which case an established connection includes two or more selectors instead of one selector as in the case of the disclosed switch train including the sglector S1.

DETAILED DESCRIPTION The system of Figs. 1 and 2 The invention having been described generally, a detailed description of its operation will now be given. The system shown in Figs. 1 and 2 will first be described, and for the purpose of this description it will be assumed that the subscriber at substation A1, Fig. 1, desires to converse with the subscriber at substation A2, Fig; 2.

Operating the line finder LFI When the subscriber at substation A1 removes his receiver, a direct-current bridge is placed across the conductors 101 and 102 of his line, clos ng a circuit for line relay 103 of line circuit LCl. Line relay 103 thereupon operates and places a ground potential at its upper armature upon conductor 105 which is multipled into the banks of all of the finders such as the finder LF1, and from which the conductor 105' extends to the bank multiple of the connectors having access to the line of substation A1. The conductor 106 is s milarly multipled in the banks of all of the line finders in the group including the line finder LS1, and branch conductor 106 exhaving access to the line of substation A1.

The ground potential placed on conductor at the upper contacts of line relay 103 identifies the line of substation A1 as a calling At its lower armature, line relay 103 closes a circuit for the second-level start relay 107, one of which is provided for each level of contacts in the banks of the finder LFl. Relay 107 operates and at its right-hand armature places a ground potential on the second-level contact in the bank of the vertical test wiper 113, thereby marking the second level as the level in which the calling line is located. Relay 107 also places ground potent al at its left-hand armature on the start conductor 130, thereby closing a circuit for the start relay of the first finder in the chain which happens to be idle. Assuming that the line finder LF1, Fig. 1, is idle, the start circuit extends to the start relay 122 therein. Relay 122 accordingly operates. When start relay 122 operates, it opens a point in the circuit of release magnet 117 and places ground potential on conductor 132, thereby preparing locking circuits for relays 123 and 124; starting the' vertical magnet 118 into operation, and lighting the filament of the vacuum tube 134 in series with resistances 116 and 115. At this point it may be mentioned that the symbols of a battery with a grounded positive pole and an ungrounded negative pole represent the regular exchange battery, of

i tor 108 sent nel-ya ns volt potential, while the vsymbols"offa. battery with the negative pole) rounded T and the positive .po'le ungroun representa plate battery, arranged to provide' pljate current for the vacuum tubes. The vacuum tubes are ass ed to be of atype requiring onlyy aboutfive'volts 0-, tential across their'filament leads, for wh ch reason tjheresistances 115 and 116 are ineluded in series with the filament of the vacuum tube 1141Ijto' givethe proper filament voltage and filament current.

,[It-will-be noted thatthe upper winding oftransfer. relay 123 "is included in circuit with the ungroundedpositive pole of the plate bat-' the plateelementof the vacuum tube 114 and tery, but only anegligible current, orno currentat all, flows through the upper wind'ng ofrelay' l23-forthe time being, as the gri element thereofjis maintained at a negative relay 125 operates andwopens the circuit of magnet 118. Magnet 1 18 thereupon deeriei gizes and opens the circuit of relay 125, whereupon relay 125 becomes deenergiz'ed and again closes the circuit of the vertical magnet.v g

ntinues until the circu t of [This action 0 p the yertical' magnet 118 is opened upon the operation of relay 123, as willbe pointed out presently.

Each time the" vertical magnet 118 'o' er- 1 Sites it raises'the wipers-111, 112, and 113 of theline finder LFl one step. Upon the first 'upwardstep of the wipers, wipers 111 and 112a're raised opposite the first level of bank "contacts, and wiper 113 is brought into env gagement with the test contact corresponding to the first level. If the'first level contains 'ing to the first level in order to terminate the vertical movement with the wipers opposite the first level. With'no ground tential on "wiper 113, the circuit of vertical magnet 118 tical magnet continues until the wipers v111 and 112 are raised opposite the second level of bank contacts, and wiper 118 is brought into engagement with the corresponding vertical test contact 109. When this occurs, the ground tential extended by .way of conduca calling line, a ground potential is encountered on the vertical test contact correspondthe first-level contact in the ban of vertical.

. ointhecontactsof relay 107tetest' as contact 109 is encountered by the test wiper 113, lac a 4 itiv'e potential on the id elem nt di thes c uum tube 114 thro gr tacts of relay 123. When the id e ement of tube 114 is thus rendered positive with respect'to the potential existing on the filament t erect, a substantial flow of current takes place from'the ungrounded tlre'plate battery, thro h I e upper winding of relay 123, and t e plate e ement of the tube 114." Relay 123 0 rates responsive to this flow of current t rough its u per (winding. .Upon so doin it closes aloc circuit for its lower win ing at its inner low-- er contacts and includi conductor 182,

grounded at thecontacts o relay 122. Relay 123 also disconnectsjthe-g 'ounded conductor 132 from'vertical magnet 1 18 and extends it to rotary magnet 119, thereby terminating the vertical testing movement with the wipers 111, 112 opposite'the calling level of bank contacts and initiating'the rotary movement to drive the wi rs 111, 112 over the contacts in'the selected evel. At its upper armature relay 123 disconnects the grid element of vacuum tube 114 from the now-grounded verr 113 and extends it to the tical test wi wiper 111, w ich is used as a'test wi r during the rotary movement of the fin er. As soon as the upper armature breaks away from its resting contact, the grid potential is shifth con-' itive pole of ed from positive to negative b virtue of the connection between the grid e ement and the negative pole of the exchan battery by way of the resistance element 1 When this oc-' curs, the current flow in the plate circuit of the tube 114 abruptly ceasesf, but this does not bring about the release 0 transfer relay .123, because this transfer relay is locked at its inner lower, contacts and lower armature to the grounded conductor 132. As, a further result of its operation, relay 123 disconnects the plate element of the tube 114,

at its inner upper armature, from its own upper winding and transfers it to the upper windin of relay 124 in order to enable the relay 1 4 to act as a test relay during the rotary movement of the finder. Y

Rotary magnet 119 and stepping relay 125 nowinter-act, causing the wipers 111and 112 to bedriven across the bank contacts in the selected level. This rotary movement con: tinues uninterrupted until wipers 111 and 112 land upon theconta'cts terminating conduc tors 105 and 106. When this occurs, vlviper on conductor 105-by relay 108 throng conv is not disturbed and the operation of, the ver- 111 encounters the ground potential tac ts of. relay 104. The positive ground poture-of relayg,.123, causing plate current to lay 124'operates and a locking circuit .for its lower wind" at its inner lower con- .tacts and including t e grounded conductor winding of line rela 132. Relay 124 also opens the circuit of the rotary magnet 119, stopping the rotary move- .ment with the wipers 111 and 112 in engagement with the bank contacts assigned to the line of substation A1.

As a further result of its operation, relay 124 at its inner upper armature transfers the plate element of the vacuum tube 104 from its own upper winding to the windingof'relay 126; connects up the conductors 128 and'129 extending to the selector S1 at its upper armature and armature 134, disconnectin the resistance 127 at the contacts contro led by its upper armature. At its lower armature, relay 124 disconnects start lay 104 by way of wiper 112 and conductor 129, through the lower windin of line relay 141 of the selector S1. Cut-o relay 104 accordingly operates and disconnects the conductors 101 and 102 of the calling line from line relay 103 and ground and extends them to the corresponding contacts in the bank of the linefinder by way of the bank-multiple conductors 105 and 106. The conductors 101 and 102 of the calling line are thus extended to trunk conductors 120 and 129 incoiningto the selector S1, the connection including the upper armatures of relays 123 and 124, and the armature 134.

The calling line is now guarded against seizure by any one of the connectors having access thereto, as a positive potential is now applied to conductor 106 through the lower 141 and conductor 129. In the line finder F1, it is to be noted that relay 121 operates in series with the lower.

winding of relay 124 when the locking circuit of relay 124 is closed at the inner lower contacts thereof. When relay 121 operates, it

removes the shunt from around the .resistance 116 and places a shunt around the resistance 115. This is for the purpose of shifting the potential of the filament 114 with respect to the exchange battery so that the potential of the filament of the tube 114 becomes more nearly the potential of the negative pole, of the battery, whereas it was formerly maintained at very nearly the potential of the positive pole of the battery so as to require'the full positive potential of the grounded pole of the battery on the grid element of the tube 114 during the testing operation to cause substantial current to flow in the plate current. By this arrangement, wiper 111 does not become sufficiently positive during the line testing operation to cause substantial current to flow in the plate circuit when the wiper is passing over the contacts of engaged lines, as the contact of an engaged line has a potential somewhere near the middle of the exchange battery because of the loop across the line at the substation and the consequent potential drop in the windings of the relay supplying current to the line. After relay 121 has-operated however, the potential of the filament of the tube 114 is more nearly the potential of the negative pole of the exchange battery, with the result that the potential normally assumed by the grid of the tube 114 by virtue of its connection to the negative conductor of the line is more positive than the potential of the filament of the vacuum tube because of the potential drop in the upper winding of the line relay of the selector S1 and re- Preparing the selector S1 for operation I In the selector S1, line relay 141 now energizes over the calling line and closes a lighting circuit for the filament of vacuum tube 151. The resistances 150 and 149 in the filament circuits are of such relative values that the filament of the tube 151 is maintained very nearly as negative as the negative pole of the exchange battery. Accordingly, the grid element of the tube 151, which is connected to conductor 128, is positive with respect to the filament and because of the potential drop through the upper winding of relay 141 and through resistance element 147. As a result of this, the plate relay 143 operates because of the flow of plate current through the tube from the associated plate battery. Upon operating, relay 143 opens a point in the circuit of vertical magnet 155 shunts the contactsof line relay 141, so as to prevent the filament circuit of vacuum tube 151 from being opened during the transmission of impulses. At its lower armature, relay 142 opens a point in the circuit of re lease magnet 157 and places ground on conductor 162, thereby completing the preparation of the selector S1 for operation under the control of the calling device CD at substation A1.

It will be understood, of course that the foregoing operations have all taken place responsive to the removal of thereceiver at access? the calling substation A1 and that these op-' erations are all accomplished within a very short time, usually considerably less than one second. 4

i Setting the selector S1 When the subscriber at substation A1 manipulates his calling device CD in accordance with the first digit in the number of the line of substation A2, Fi 2, the digit 3 in v the present case, the cal ing loop is interrupted three times at the contacts of the call ing device CD upon the return of the calling device to its normal position Each time 15. the loop is interrupted, conductor 101 of the callin line and conductor 128 incoming to the sol ctor S1 become negative to the full extent of the negative ole of the exchange battery, and momentari y more negative than that because of the self induction of line relay 141. Atthe same time,-the positive conductor 102 of the calling line and conductor 129 incoming to the selector S1 become more positive than formerly, although this is not of any particular importance at present as the control of theselector S1 is accomplished 4 over conductor 128.

Each time conductor 128 is rendered negative by the opening of the loop at the calling substation, the current flow throu the plate circuit of tube 151 is interrupted y the action of the gridelement by virtue of its connection with conductor 128, whereupon relay 143 is momentarily deenei'gized, opena ing the circuit of the slow-acting release re= lay 142. Relay 142, however, does not fall back when its circuit is opened onl momentarily. Each time it falls back, re ay 143 at its lower armature extends ground potential from the grounded conductor 162, by way of contacts of relay 144, and series relay 145, to

vertical magnet 155.- By the operationof. vertical magnet 155 the wipers 158 and 159 are raised step byl ste until they come to rest opposite t e t mains operated throughout the vertical movement, maintaining the circuit of relay 144 v I When the slow-acting series'relay 145 falls back at the end of the'impuls'e series, it completes a circuit from the grounded conductor I the grid element oil the tube thro ird evel of bank contacts. 4

.. Relay is operated responsive to the firstimpulse of current'delivered to vertical magnet' 151. Being slow acting, relay 145 re-.

open after elf-normal vcontacts 153 haveov 'rflow contacts 154, forthe transfer relay 144. Relay trolled by theQrestm contact offthe lower armature of plate re ay.143, from vertical magnet 155 to the switching relay 146, At

1 through the ofi-nor'mal contacts 153 and 144 thereupon operates and transfers the now-open operating circuit, conits upper armature, relay 144 extends ground potential to rotar ma net 156 from conductor 162 byway, o the ower armature ofthe operated relay 143, whereupon the self-interrupting rotary magnet 156 starts to o erate 'in a buzzer-like manner to drive 0 wipers 158 and 159 across the third level of bank contacts in search of an idle trunk line.

Asa further result of its operation, relay 144, at its upper. armature, disconnects the grid element of tube 151 from the incoming conductor 128 and shifts it into connection with wiper 158 so .as to enable the negative trunk conductors of the trunk lines 1n the third level to be tested to determine the busy or idle condition of these trunk line s. Any engaged one of the trunk linesin the third level is identified by the relatively positive potential on its upper conductor owing to the potential drop in the relay winding supplying current to said conductor, while the idle trunk lines in the level are identified by the full negative potential of the exchange battery. f As a result, plate relay 143 remains operated until anidle trunk lineis reached, which trunk line it will be assumed is the one comprising conductors 150 and 161 and extending to the connector 01, Fig. 2. When such idle trunk line is reached, the current flow inthe late circuit of tube 151 ceases because of the negative potential placed on h wiper 158; .When this occurs, relay 143 falls back and opens the rotary magnet circuit at its lower armature, at the same time closing a circuit through the lower armature of. the

operated relay 1 44 for switching relay 146. Relay 146 thereupon operates and closes a locking circuit-for itself to the grounded conductor 162 at its inner lower armature, at the same time opening its initial circuit, At its upper and 'lower'a'rmatures, relay 146 disconnects the incoming conductors 128 and- 129 from the windings of line relay 141 and extends them by way of wipers 158 and 159 to the conductors 160 and 161 extending to the connector 01. The current which-flows from now on over the calling line is obtained over relay 201 of the connecter C1 and resistances 211. and 212, resulting-in a lowering ofthe potential on conductor 126', thereby marking the trunk line extending to the connector 01 as busy. p I

As a result of the transfer of incoming conductor 128 to wiper 158 by the upper armatune of relay 146, the tube 151 is 'again conductors 160 and 161'and through the linethe circuit of release relay 142 so as to maintain the release relay operated. The circuit I of the rotary magnet 156 is not again closed because ground potential has been removed from the lower armature of the plate relay at the middle-lower contacts of relay 146. At the same time that ground potential is removed from-the lower armature of relay 143, the circuit of transfer relay 144 is opened, whereupon relay 144 falls back and transfers the grid element from wiper 158 to incoming conductor 1 28 again. This operation has no particular utility at this time, the circuit of relay 144 merely being arranged so that it is opened when relay 146 operates so as to reduce current consumption.

In the line finder LFl, it will be recalled that the control relay 126 is now connected in the plate circuit of tube 114 by way of contacts of the operated relays 123 and 124 and is maintaining release relay 122 operated. During the transmission of impulses to the selector S1, the flow ofplate current inthe tube 114 is terminated each time the circuit is opened at the calling device, causin relay 126 to fall back each time the impu se cir-' cuit is o ned and to reoperate each time the im u e circuit is closed. Relay 122 does not fa 1 back at this time because it is slow acting.

Preparing the connector 01 for operation In the connector C1, line relay 201 energizes over the calling line and over the incoming conductors 160 and 161 when this connector is seized by the selector S1, preparing a holding circuit for release relay 203 at its power armature and closing a li hting circuit for the filament of vacuum tu 213 at its upper armature. The filament lighting circuit for tube 213 includes the high resistanee 213 and the low resistance 214, and these two resistances are so proportioned that the potential now impressed on the grid element of the tube is positive with respect to the potential on the filament, resulting in a flow of plate current from the ungrounded positive pole of the plate battery through plate relay 204. Relay 204 operates and closes a circuit for release relay 203, whereupon release relay 203 operates and places ground potential on locking conductor 226 at its lower armature, thereby preparing the switch for operation as well as preparing locking circuits for relays 206, 208, and 210, and completing a lockm ta ts of relay 201.

circuit for itself through the lower con-.

Setting the connector 01 When the subscriber at substation A1 dials the second digit in the number of the line of substation A2, the grid element of the vacuum tube 213 is rendered highly negative a number of times momentarily, thereby momentarily stopping the flow of plate current a corresponding number of times. Each time the plate current is stopped, plate relay 204 falls back and closes atits lower armature a circuit from grounded conductor 226 by way of contacts of relay 210 for the vertical magnet 216. By the operation of the vertical magnet 216, the wipers 222 and 223 are raised step by step until they come to rest opposite the desired level of bank contacts. Slowacting series relay 205 operates upon the first vertical impulse and remains operated throughout the series of impulses. Relay 205 makes a circuit change so as to maintain its own circuit and that of the vertical magnet 216.intact after off-normal springs 218 and 219 have shifted, as they do upon the first vertical step of the switch.

At the end of the series of impulses, relay 205 falls back and transfers the operating circuit through off-normal contacts 219 and its own lower armature and resting contacts to the rotary magnet 217 by way of contacts of the two-ste relay 208.

When the nal digit is dialled, the impulses delivered by plate relay 204 are transmitted to the rotary magnet 217 through olfnormal contacts 219, and contacts of relays 205 and 208. By the operation of magnet 217 the wipers 222 and 223 are rotated step-bystep over the bank contacts in the selected level until they come to rest upon the set of bankcontacts assigned to the line of substation A2. Relay 207 operates when the first impulse is transmitted to rotary magnet 217. Being slow-acting, relay 207 remains operated throughout the series of impulses. At its upper armature, relay 207 operates slowactin relay 209 to prepare a circuit for switc ing relay 210 and to prepare to shift the grid element of the tube 213 into association with wiper 223 to enable the called line to be tested. At its inner armature, rela 207 closes a circuit through the upper win ing of the two-step relay 208. The up r winding is relatively ineflicient and is a le, therefore, to operate only the first-step innerlower armature of the relay, thereby closing a locking circuit for relay 208 through both windings in series. This locking circuit is ineffective, however, as long as the initial circuit is established, as the same potential is connected to both terminals of the lower winding.

When relay 207 falls back at the end of the rotary movement, it opens the circuit of slow-acting relay 209, but relay 209 does opens theinitialcircuit of the u per winding of relay 208, whereupon bot windin s of the relay, ener ize in series, causing t e relay to operate fully. Upon fully operating, relay 208 transfersthe now-open operating circuit at. its middle lower armature from rotary magnet 217 to switching relay 210" by way of contacts of relay 209; prepares a busy-tone circuit at its lower armature; and at its ugipfr armature transfers the grid element 0 t e vacuum tube 213 from the incoming conductor 160 through the upper armature of the still-operated slow-acting relay 1 209 to the wipr 223, nowstanding on the lower bank contacton the line of substation A2.,

Thecalled line m The operation now to take place depends .upon whether the line of substation A2 is busy or is idle when called. If the line is busy, relay 233 is in an energized condition responsive to an application of positive potential thereto over conductor 232 (if the line is busy as a called line), or over conductor 232 (if the line is busy as a calling line). In this case, the positive potential encountered on conductor 232 or wiper 223 main: tains apositive condition on the grid element of vacuum tube-213 and the plate relay 204 is maintained operated continuously. Under this condition, switching relay 210 is unable to operate. Relayr 208, therefore, re-

, mains operated. Then, when relay 209 falls way of the lower contacts of relay 208. The

grid element of the tube 213'is transferred back to the incoming conductor 160 at the upper armature of 'relay209, thereby, again placing the platerelay 204under the con-v trol of the calling line.

The busy-tone current applied "to the low er talking conductor from the transformer 254 is transmitted back to the callin line as a characteristic busy tone, informing he subdescribed.

scriber ofthe busy condltion of the desired line. Upon hea'ringthis busy tone, the calling subscriber is expected to replace his'recelver and bringabout the release of the operated switches in a manner to be hereinafter The called Zine idle f 1 Assuming now that the line of substation .A2 is idle when called, plate relay 204 falls back when the'grid element of tube213.is'

transferred at the upper armature of relay" 208 to wiper 223 by way of contacts of relay 209, as wiper 223 encounters a negative potential on conductor 232, transmitted thereto from the negative pole of the exchange battery b way of the cut-off relay of the called line. TVhen relay'2'04 falls back, it closes, at its lower armature, a circuit through off-nor mal contacts 219, contacts of the restored relay 205, and contacts of the operated relays 208 and 209 for switching relay 210. Relay 210 thereupon operates and closes a locking circuit for itself at its inner-lower armature; extinguishes the filament of the tube 213 at its inner -upper-ar'mature; and at its upper and lower armatures it completes the connections to the conductors 231 and 232. Relay 209 falls back after a short interVal,-responsive to its circuit having beenopenedat t e upper contacts of relay 207.

Relay 204 is now deenergized because the filament of the tube 213 has been extinguished at the inner upper contacts of rela 210, but relay 203- remains operated throug its self-locking circuit including the lower contacts of line rela 201.

Ground potential 1s now placed on conductor 232 by way of the lowercontacts of ringcut-ofi relay 206 and the lower armature of of t is relay to the called substation, by wayof the upper talking conductor. The return path for theringing current is b way of the lower talking conductor of t e established connection to ground through the lower contacts of relay 206. Owingto the'in- 'clusion of the 'condenserin the ringer circuit at the called substation, no direct current flows over the ringing circuit for the time being. Ring-but-ofi' relay 206, being of standard slow-acting construction, does not.re-

spond to the alternating current.

- "When the subscriber at substation A2responds to the ringing of his bell by removing his receiver, a directcurrent bridge is ,placed across the conductors of his line,

whereupon direct current flows over, the called'line through the upper winding of rela 206, bringing about the operation'of this re ay. Upon operating, relay 206 closes a locking circuit for, its lower 'windingag it inner lower contacts to'the grounded lockrelay 210, rendering the called line busy to the other connectors and operating cut-ofi' 'ing conductor 226. At its upper and lower armatures, relay 206 opens the cuit and connects the wipers 222 and 223 to the windings of the back-bridge relay 202, through which talking current is supplied to the called line. The cut-ofi relay 233 of the line circuit LC2 is maintained operated in series with the lower winding of back-bridge relay 202. V v

Back-bridge relay 202 now operates over the called line and shunts the resistance element 211 and 212, thereby increasing the current flow over the calling line so as to better energize the transmitter at the calling substation for conversational purposes. This increase of current flow brought about by cutting out the resistance elements 211 and 213 may be utilized in any-desired manner to effect supervision, which includes operating an individual line meter at the exchange or at the calling substation, or the operation of a supervisory relay in a cord circuit in case the call is coming from an operators switchboard (the switchboard of a calling P. B. X. operator for example).

The conversation may now take place between the two substations A1 and A2 as desired, the talking circuit being outlined by the "heavy conductors.

It will be noted that the talking conductors are reversed between the windings of the front-bridge line relay 201 and the windings of the back-bridge battery feed relay 202. This reversal of the connections has no efiect upon the flow of direct current, as it is made in a portion of the talking circuit in which direct current does not flow because of the inclusion of the talking condensers between the front and back bridges of the connector. By this arrangement, however, the talking circuit is more nearly balanced against foreign disturbances, as the upperincoming talking conductor of the connector is connected to the lower wiper of the connector and the lower incoming talking conductor of the connector is connected to the upper wiper thereof. In this'way, the cut-off relay 104 of the calling line and the cut-off relay 223 of the called line are effectively placed on opposite sides of the talking circuit, so that any tendency of one of these relays to unbalance the talking circuit is largely compensated for by the connection of the other relay' on the opposite talking conductor. It is appreciated, of course, that this balance is not, absolutely perfect, even though the two relays may be identical in characteristics, because of the relatively slight resistance in the portion of the talking circuit including theautomatic switches and the condensers in the connector. It is plain, however, that a much greater degree of balance is obtained with the reversal above pointed out at the connector than would be obtained if both cut-off relays were connected to the same continuous talking conductor.

Releasing the established connection After the conversation has been carried on and terminated, the two subscribers replace their receivers. When the receiver is replaced at substation A2, back-bridge relay 202 falls back and again inserts the resistance elements 211 and 212 in circuit with the line relay 201, so as to give disconnect supervision when desired.

When the receiver is replaced at substation A1, the circuit of line relay 201 of the connector C1 is opened and this relay falls back, as the current flowing over the lower talking conductor throu h its lower winding and through the big resistance cut-off relay 104 is insutficient to maintain it operated. At the same time, the potential of the upper talking conductor becomes more negative, rendering the grid element of the tubes 114 and 151 negative and stopping the flow of plate current in these tubes to bring about the restoration of plate relays 126 and 143, with results to be explained more in detail hereinafter. In the connector C1, relay 201 opens the circuit of slow-acting relay 203 at its lower armature, and this relay shortly falls back, as it is no longer maintained operated by plate relay 204, because the control of the plate relay 204 has been terminated upon the previously described opening of the filament circuit u on the operation of switching relay 210. 203 falls back, it removes ground potential from lockin conductor 226, permitting the locked up re ays 206 and 210 to restore. Re-

lay 203 also closes a circuit through offnormal contacts 221 for release magnet 220. By the operation of the release magnet 220 the switch is restored to its normal position, whereupon the release-magnet circuit is opened at off-normal contacts 221.

In the selector S1, when plate relay 143 falls back it opens the circuit of the associated slow-acting release relay 142, which release relay falls back shortly thereafter. At its upper armature, relay 142 extinguishes the filament of the tube 151, and at its lower armatureit removes ground potential from conductor 162 to permit relay 146 to restore, at the same time closing the release-magnet circuit. Release magnet 157 thereupon 0perates to. restore the selector S1 to its normal position, whereupon the release-magnet circuit is opened at off-normal contacts 152.

In the line finder LF1, release relay 122 falls back shortly after its circuit is opened at the contacts of plate relay 126. Upon falling back, relay 122 removes ground potential from conductor 132, extinguishing the filament of the tube 114 and permitting the locked-up relays 121, 123,and 124 to fall back. The release ma net 117 0 rates at this time and restores t e line fin er LFl to its normal position, whereupon the releasehen relay the circuit of t magnet circuit is opened at ofl-normal contacts 122.

The entire connection is now released, the apparatus involved. therein is in readiness to be used in setting up subsequent connections.

the exchange battery on conductors such as 160 in order to bring about the deenergization of plate 143 to close a circuit for the seizingo'r switching relay 146. It isto be noted that relay 146. is stiflly adjusted, as indicated by label applied in the drawin This characteristic is given to the relay an increased tension of the restorin sprlngs, and it has the effect of delaying t e operation of the relay for a slight interval after e relay is closed. By this arrangement, the o eration of switching relay 146 is prevente when its circuit is closed only momentarily during trunk hunting, in the event that the plate relay 143 is momentarily deenergized during the trunk-hunting time, in case test wiper 158 hap ensto arrive on a bus trunk just as t e impulse springs are bemgopened at the calling devlce to transmit an impulse. This, it will be appreciated, is 'a rare occurrence, but the above stated provision has been made hecausethe coincidence is theoretically possible. The stifi adjustment of relay l46 is' useful also in delaying the seizure of a selected trunk long enough to permit the previously established connection to be released in case a trunk happens to be seized (which it very rarely does) just as the receiver is being.

replaced at the calling substation.

The system of Fig 3 Referring now particulafl'y to Fig. 3, the modification disclosed therein will be described. In this modification, balancing coils are employed in the finder and connector, in place of the reversal in the talking conductors of the connector C1 of Fig. 2, to offset the unbalancing tendency of the cut ofl relays of the calling and called lines; standard reverse-battery supervision is em: ployed in Fig; "3 in place'of thehigh-resistancesupervision in the modification of Figs. 1 and 2; and the selector S2 and connector C2 of Fig. 3 has been suitably modified so that the connector is continuously guarded -against seizure by maintaining altered potential on its lower incoming conductor from the time the connector is seized until it is completely restored to normal .conditio n.

Except for the difierences above noted, the circuit arrangementsof the modification of Fig. 3 ma be the same as the circuit arrangements of igs. 1 and 2. r v

More in particular, the line circuit L03 of the substation A3 is similar to the line circuit LCl'of the substation Al; the line finder LF2 is similar to the line finder LFl,

Fig. 1, except for the addition of the balancing coil 37 1, connected to the upper talking conductor to offset the cut-off relay .(such as 304 of the line of substation A3? ing line; the selector S2 is simi ar to the selector S1 of Fig. 1 except for the addition of the'lower pair of contacts to transfer relay 344, the shunt resistance 378, and the shiftof the calling of the test conductor from wiper 358 to wiper 359; the connector C2 is similar to the connector G1 except for the modified guarding circuit involvln armature 380 o elay 303, the associated 0 -normal spring 381 and its contacts, the resistance element 382, the addition of coil 379 and contacts to connect it to the upper talking conductor to balance the cut-off coil of the called line, and the substitution of reversing contacts on the backbridge battery-feed relay 301 instead of the resistance-shunting contacts of the corresponding relay 202, Fig. 2.; and the line circuit L04 is similar to the line circuit LC2 of substation A2.

It is to be noted that the relays and other .devicesof Fig'. 3 corresponding to similar relays and devices of'Figs. 1 and 2 have been given the same tens and units di ts in their reference characters, which are istinguished in Fig. 3 from the corresponding ref-- erence characters applied to Flgs. ,1 and 2 only in that the first di it of each of the reference characters of ig. 3 is the di 't 3, whereas in Fig. 1 the first digit of eac I reference character is the digit 1, and the first digit of each reference character a plied in Fig. 2 is the digit 2. Certain of t e reference characters 1n Fig. 3 will not be specifically mentioned in the descriptlon to follow,

but these reference characters are applied in Fig. 3 in order that the circuits of F1g. 3 may be more readily comparedwith the c rcuits shown in Figs. 1 and 2.

In order to enable the modification in Fig. 3 to be understood more fully, the operation of-the apparatus of Fig. 3 in setting up a connection'rom the line of substation A3 to the line of substation A4 will now be explained.

Setting up a connection When the receiver is removed at substation A3, line relay 303 is energized over conductors 301 and 302, whereupon relay 303 grounds .the associated'start conductor'at its lower armature and places theidentifying ground potential upon conductor 305 at its up er armature. When this occurs, a line fin er, the line finder LF2, for example, is started and operates in the manner described in connection with the line finder LFl, Fig. 1, to raise its wipers to the level of bank contacts containin .the calling line and to rotate them over t e bank contacts in this level until the contacts of the calling line are reached. When this occurs, the calling line is seized by the line finder LF2, whereupon relay 324, corresponding to relay 124, Fig. 1, operates to extend the line of substation A3 to the selector S2, automatically connecting the retard coil 371 to the upper conductor of the calling line to balance the cut-off relay 304 which is now operated over the lower talking conductor in series with the lower winding of line relay 341 of the selector S to cut oil the line relay 301 and the associated ground connection.

In the selector S2, line relay .341 energizes over the conductors 328 and 329, closing the li hting circuit for the filament of tube 341, w ereupon plate relay 342 operates, followed by the operation of release relay 342, preparing the selector for operation.

When the first digit in the number is dialled, plate relay 343 responds to close the circuit of verticalmagnet 355 by Way of series relay 345 each time the impulse springs separate at the calling device. The wipers 358 and 359 are thereby raised opposite the desired level of bank contacts. As soon as relay- 345, which is maintained operated throughout the vertical movement, falls back, relay 344 operates through ofl-norma'l contacts 353 and overflow contacts 354, transferring the operating circuit to switching relay 346 and connecting up rotary magnet 356. At its -upper armature, relay 344 transfers the grid element of the vacuum tube 351 from the incoming conductor 358 to the positive line wiper 359 so as to enable the trunk lines in the selected level to "be tested by wiper 359. Each trunk line in use is marked by a ositive potential on the conductor thereof corresponding to conductor 361, with the result that relay 343 is maintained operated until an idle-trunk line is reached, which is marked by a negative potential on the lower conductor thereof.

When an idle trunk line is reached, the trunk line extending to the connector C2, for example, the negative potential encountered by wiper 359 on conductor 361 through resistance element 382 is transmitted to the grid element of the tube, and stops the flow of plate current, whereupon plate relay 343 falls back and opens the circuit of rotary magnet 356, at the same time closing a circuit for switching relay 346. Switching relay 346 operates to seize the connector C2 by way of conductors 360 and 361, at the same time removing ground potential from the lower armature of plate relay 342 and opening the circuit of transfer relay 344. Trans- 65 fer relay 344 does not fall back immediately line relay 302.

ringing current is applied.

because of the self induced current circulating through its winding and through the shunting resistance element 378. As long as it remains operated, relay 344 maintains the lower winding of line relay 341 connected to incoming conductor 329 at its lower armature, thereby maintaining the cut-off relay 304 of the calling line operated and maintaining a circuit closed over the calling line and over the upper talking conductor for 7 the upper winding of line relay 302 of the connector C2.

In the connector C2, responsive to the operation of line relay 302, the associated vacuum tube (not shown) is lighted, whereupon the plate relay operates as explained in connection with the connector C1 and closes a circuit for the release relay 303. Release relay 303 at armature 380 disconnects the incoming conductor 361 from the resistance element 382 and connects it to the lower winding .of line relay 302, by way of contacts of back-bridge relay 301. The two windings of line relay 201 may now remain energized in series over the calling line, and 9 the cut-off relay 304 may now remain energized in series with the lower winding of In the selector S2, relay 344 falls back after a sufficient time interval has elapsed to allow the above-described operation to take place in the connector. When it does so, it transfers the grid element of the vacuum tube 351 from wiper 359 to the upper talking conductor 328, and at its lower armature it I disconnects the lower windin of line relay 341 from conductor 329, as t is connection is no longer needed or desired after the above-described operation of release relay 303 of the connector C2. It may be pointed out, however, that in case the calling subscriber commences to dial the next digit before relay 344 falls back to disconnect the lower winding of line relay 341, no harm results as the control of the connector C2 is efiected over' the upper talking-conductor,

as will be understood upon reference to theconnector C1, Fig. 1.

Responsive to the dialling of the second and. third digits in the desired number, the connector C2 operates as described in connection with the connector G1, Fig. 2, to raise its wipers opposite the desired level of bank contacts and rotates them into engagement with the contacts assigned to the line of substation A4. The called line is tested in the hereinbefore described manner and is seized in case it is found to be idle, following which When the called subscriber responds, the ring-cut-ofi relay, corresponding to relay 206 of the connector C1, operates to open the ringing circuit and complete the talking circuit, whereupon back-bridge relay 301 operates over'the called line and reverses the in- 139 relay 302, thereby reversing the direction of coining conductors 360 and 361 as regards their connection with the windings of line current-flow over the calling line, as is done now in most standard step-by-step automatic telephone systems, and for the same purpose.

It is to be noted that cut-off relay 304 is now maintained operated throu h the lower winding of line relay 302, as be ore, but now over the upper talking conductor of the established connection and over the two conductors of the calling line in series. The selector S2 and the line finder LS2 are held operated in the same way as before, because the potential on either of the two talking conductors during conversation is more positive" than the potential of the filament, permitting the plate relays of these switches to remain opera-ted.

Releasing the established connection The release of the connection set up through the paratus shown in Fig. 3 will now be explained. In case the subscriber at substation A4 replaces his receiver first, backbridge relay 301 falls back and reverses the connection between line relay 302 and the in comin conductors 360 and 361 back to normal.

the selector S2 release in the manner described in connection with the line finder and the selector S1, Fig. 1.

In the connector C2, the release of the connection is efiected when release relay 303 falls back responsive to its circuit being opened by line relay 302, and as explained in connection with rela s 203 and 201 of the connector 01, Fig. 2. en release relay 303 falls back t pe the incoming. conductor 361 winding of line rela 302, but it does not at once connect it to't e negative pole of the exchange battery by way of the resistance element 382, but connects it to ground, hecause'ofi-normal spring 381 is in its operated position. By this arrangement; the positive guarding potential is maintained on conctor 361 until the restoration of the connector is completed, whereupon off-normal s ring 381 is restored, reconnecting negative, i le-indicatingpotential to conductor 361 by way of the resistance element 382.

JOoer/low at the selector S1 In case the selectorSl is unable to find an idletrunk line in the selected level, when its rotary movement is initiated at the end of the dialling of the first digit, the rotary movement continues until all the trunk lines have been tested and the wipers have passed off the last set of bank contacts in the level.

When this occurs, the overflowcontacts 154' are separated by theusual cam provided on the shaft of'the switch, whereupon relay 144 falls back and opens the circuit of rotary n'this case, when the calling subscriber replaces his receiver the line finder LF2 and rate the release magnet it disconnects from the lower receiver is replaced at the calling substation.

When the calling subscriber replaceshis receiver, relay 143 falls back and opens the circuit'of release relay 142, at the same time sending an impulse 0 current to vertical magnet 155 and relay 145. This magnet and.

this relay operate at this time, but the operation is merely incldental and serves no useful purpose. A moment later, relay 142 /falls back and opens the vertical-magnet circuit at its'lower armature, at the same time closing the release-magnet circuit at the resting contacts of its lower armature, and extinguishing the filament circuit at its upper armature.

.The release of the selector takes place in the usual manner.

- Overflow at the'selector 82 In case the selector S2 is unable to find an h idle trunk in the selected level, the rotation continues as described in connection with the selector S1 until the overflow contacts 354 operate, whereupon relay 344 falls back to open the circuit of the rotary magnet to stop the rotary movement and totransfer the grid element of the tube 351 back under control of conductor 328. It is to be noted that-the falling back of. the relay 344'is not delayed in this case, as-the resistance element 378 is disconnected from relay 344 when the overflow contacts 354 separate.

. Referring again to ,theconductor C2, Fig.

3; it may be pointed out that-the spring con-.

tacts by means of which the reversal of line current is efl'ected when relay'301 operates vmay be bent or adjusted so that they are not affected by the operation of the relay, thereby doing away with the current reversal. It will be understood, of course, that the reversal of current is not needed or desired in many installations. v

It may be pointed out further that the cont nuous guarding feature of the conduc 'tor C2, and involving resistance element 382, together with off-normal springs381 and its contacts,'may be applied in the same manner to the conductor C1. Fig. 2, in which case the selector. S1 would preferably be replaced by selector S2; the selector S2 tests-over the lower talking conductor in place of the upper talking conductor, and is otherwise especially adapted to operate 'in conjunction with a connector having the guarding feature I above set forth.

What is claimed is: a 1. In combination, an automatic switch, a

line .extending to said switch, a vacuum tube having its grid controlled over said line, a plate circuit for said tube including a relay, 'primary :and secondary motor magnets con- 5 trolled by said relay, and means for shifting the control from the primary magnet to the secondary magnet.

2. In combination, an automatic switch, a line extending to said switch, a vacuum tube having its grid controlled over saidline, a plate circuit for said tube including a relay, a primary motor magnet, a circuit controlled by said relay for directively operating said magnet, a secondary motor magnet, and a circuit for automatically operating said secondary magnet also controlled by said relay. 3. In combination. an automatic switch, a line extending to said switch, a vacuum tube, 'a grid in said tubenormally controlled over said line, primary and secondary operating magnets successively controlled by the late circuit of said tube, and means for shiiiing the control of said grid from the line to a wiper of the switch during the operation ofsaid secondary magnet.

4. In combination, an automatic switch, a line extending to said switch, a vacuum tube, a grid in said tube controlled by impulses transmitted to said switch over said line, a primary motor magnet controlled by said tube under the control of said grid to select a group of lines accessible to said switch, means effective upon the selection of the group for causing said grid to be controlled over a wiper of said switch, and a secondary magnet controlled by said tube for operating the switch to select an idle line in the selected group.

5. In combination. an automatic switch, a line extending to said switch, primary and secondary operating magnets for said switch, a vacuum tube for controlling said magnets and means for connecting the grid 0 said tube to said line during the operation of said primary magnet and to a switch wiper during the operation of said secondary magnet.

6. In an automatic switch, a magnet for causing the switch to hunt for an idle line, a vacuum tube, a plate relay controlling said magnet, and a grid connected to a wiper of said switch. 7. In an automatic switch, a magnet for causing the switch to hunt for an idle line, a vacuum tube, a plate relay controlling said magnet. and a grid circuit including lines successively tested by said switch.

.8. In an automatic switch, a magnet for causing the switch to hunt for an idle'line, a .vacuum tube, a plate relay controlling said magnet, a id in said vacuum tube controllingsaid pfii te relay, and means for connecting said grid to a talking wiper of said switch during the hunting movement thereof.

9. In a switching system, a group of lines, an automatic switch having a contact maker,

means for operating said contact maker to make connection with said lines successively, means in said automatic switch arranged to be controlled to indicate the busy or idle condition of one of said lines, and means including an electron-discharge device having a control element connected to said contact maker for controlling the last named means.

10. In a telephone system, a group of subscribers lines each comprising a pair of talking conductors, a two-wire switchboard multiple for each of said lines, switching devices for extending connections to said lines. and testing apparatus associated with said switching devices including electron-discharge devices for determining the busy or idle condition of a line when connection is to be made thereto.

11. In a switching system, a group of trunk lines, a hunting switch having access to said trunk lines, means for operating said switch into association with said trunk lines successively, a vacuum tube in said switch having its control element connected to the test wiper of said switch, and means controlled by said vacuum tube in accordance with the busy or idle condition of the tested trunk lines for causing the selector to pass over the busy lines and make connection with the first idle one.

12. In a switching system. a selector switch having access to a plurality of groups of trunk lines, said selector being arranged to respond to directing impulses indicative of a desired group and select such group and to then rotate over the trunk lines of the group. and means including a vacuum tube arranged to control said selector through its plate circuit and having a control element for controlling the current flow in the plate circuit for causing said selector to respond to the directing impulses and for causing said'selector to rotate over the trunk lines in the selected group until an idle trunk line is reached.

13. In a switching system, an'automaticselector switch having access to groups. of

trunk lines, said selector switch'havingan incoming conductor over which it is controlled and a normally disconnected test wiper through which it makes contact with the terminals of the trunk lines, a wiper-connecting relay for connecting up said wiper to the incoming conductor after the switch has been positioned, primary and secondary operating magnets for operating said switch in its primary and secondary movement, respectively, a vacuum tube having a plate circuit, and means for controlling said primary and secondary operating magnets and said wiper-connecting relay successively through the plate circuit of said vacuum tube.

'14. In a-switching system, a group of lines, an automatic switch arranged to contact with said lines successively, said lines having differing potentials thereon, depending upon whether they are in use or idle, an electronsivelyto regulate the flow of electrons in said device according to whether any trunk line being tested is busy or idle.

15. In a switching system, a group of lines, an electron-discharge device having a control element arranged to influence the electronflow within said device by altering the space charge within the device, and means for connecting said control element to said lines successively to cause the electron emission to be regulated by the respective potentials of the successively engaged lines.

16. In a switching system, a group of lines, an electron-discharge device having a control element arranged-to influence the electronflow within said device by altering the space charge within the device, means for connecting said control element to said lines successively to cause the electron emission to be regulated by the respective potentials on the successively engaged lines, and meanscontrolled by the discharged electrons when a line having a predetermined potential thereon is reached foristopping the testing operation.

17 In a switching system, aoup of lines, an electron-discharge device aving a control element arranged to influence the electron-flow within said device by altering the space charge within the device, means 'for connecting said controlelement to said lines successively to cause the electron emission to be regulated by the respective potentials on the successively engaged lines, means con-- trolled by the discharged electrons when a line having a predetermined'potential there-' on is reached for stopping the testing operation, and for seizing such line and altering the potential thereon.

18. In a switching system, a group of lines, any one of said lines having a given predeterniined potential thereon in case it 1s idle and a different, predetermined potential thereon in case it is in use, an electron-discharge device, said device having a control element arranged to influence the volume of the discharged electrons by regulating the space charge within the device means ineluding an automatic switch for associating the control element of said electron-discharge device with said lines successively, and means controlled through said electron-dis.- charge path and responsive to the control element of said device being connected to an idle line for stopping the movement of said automatic switch and for causing the idle line to be seized andmarked engaged.

19. In a switching system, an automatic switch having ad'isconnected wiper through which connection is made with a line in any one of a plurality of groups, said switch having a primary movement and a secondary movement, a control conductor incoming to said switch, a vacuum tube having its control element connected to said incoming'control conductor,'a primary ma net controlled through the plate element 0 said vacuum tube under the "control of the control element responsive to impulses received over said control conductor to set the said wiper on a group of lines, means thereupon effective for shifting the plate element of said vacuum-tube from said primary magnet to the secondary magnet to enable the secondary magnet to operate under the control of the vacuum tube to locate said wiper onto -a desired line in the selected group, a wiper-connecting relay, means eflective after the switch has been positioned onan idle line for operating. the wiper-connecting relay under the control of the plate circuit on the vacuum tube, and means for shifting the control element of the vacuum tube to the said disconnected wiper toefiect such operation.

20. In an automaticswitch, a vacuum tube for controlling the switch, a grid in said tube on which different potentials are impressed, a plate circuit for controlling said switch responsive to such potentials, and means for automatically adjusting the filament otential of said tube in order to render sai plate circuit operative.

21. In an automatic switch, a vacuum tube for'controlling successive operations of said switch, said tube including'an emitting element which is included in the grid and plate circuits, and means responsive to one operation of said switch for altering the potential of said emitting element to prepare the switch to perform the next operation.

22. In an automatic switch, two talking conductors, a vacuum tube for controlling v successive operations of said switch, a gri element in said tube connected to one talking V conductor while the switch is performing one operation, and means for connecting said grid to the other talking conductor while the switch is performing another operation.

23. In a telephone system, an automatic switch for extendinga full metallic talking circuit, a vacuum tube, means in the plate circuit of said tube for controlling the operation and holding of said switch and'also the testing function thereof, a grid in said tube connected, to one side of the talking circuit, and means for temporarily shifting the grid connection to the-other side of the talking circuit while said testing is being performed.

24. In an automatic switch, operating means for selecting a line, means includi a vacuum tube for testing a selected line aiid forcontrolling the r'elease-of the switch, a

connection from the grid of. said tube to one function 1 the primary movement, a relay for stopping the secondary movement, a vacuum tube, and means for connecting'said relays successively in the plate circuit of said tube.

27. In a finder switch, a vacuum tube, means controlled over the plate circuit of said tube for causing the switch to locate a desired line, and means controlled over said plate .circuit thereafter for maintaining said switch in connection with said line.

28. In a telephone system, a subscribers line terminating at a telephone exchange, a section of said line in the exchange being normally disconnected from the main portion thereof, a linerelay normally connected to the main portion of said line in the'exchange, said line relay being responsive to a calling condition on said line to place a distinctive potential on one talking conductor of the disconnected section of the line, a line finder having .access to said line and to other similar lines and arranged to search over the disconnected sections of said lines for any calling line, and means in said finder responsive to the said potential placed on a talking conductor of the disconnected section of the line for causing the finder to stop in association with said line and to make connection therewith by way of said disconnected section.

/ 29. In a telephone system, a subscribers line terminating at a telephone exchange, a section of said line in the exchange being normally disconnected from the main portion thereof, a line rela normally connected to the main portion of s aid line in the exchange, said line relay being responsive to a calling'condition on said line to place a distinctive potential on one talking conductor of the disconnected section of,the line, a line finder having access to said line and to other similar lines and arranged to search over the disconnected sections of said lines for any calling line, means in said finder responsive to the said potential placed on a talkin conductor of the disconnected section 0 the line for causing the finder to stop in association with said line and to make connection therewith by wayof said disconnected section, a cut-ofl relay connected to the other conductor of said disconnected section, and means for operating said cutofi "relay responsive to the said finder making a finder switch, a grid circuit for 581d testing wiper during the .talking conductors, each line being divided into two sections, each line comprising a main section and-a normally disconnected sectlon, means for connecting the two sections of any line together while the lineis in use, circuit connections for maintaining a characteristic potential on the conductors of said line while it is in .use, and means controlled over the main portion of said line from a calling substation thereon for placing a different characteristic potential on one of the disconnected talkin conductors of paid line to identify said line as a calling 31. In a telephone system, a group of telephone lines, a finder switch having access to said lines and arranged to be started into operation res onsive to a calling condition on any one 0 said lines to test the lines successively until the calling line is found, means also responsive to the said calling condition for placing a characteristic potential on one of the talking conductors of the calling line characteristic of the calling condition, and means including a vacuum tube in said finder switch controlled over the talking conductors of said lines as they are successively tested for terminating the operation of said finder switch when it reaches the line on which the calling condition exists and for making connection with said line.

32. In a telephone system, a finder switch having access to groups oflines and arranged to be started into operation to find a calling line in any group, said finder switch having a group-testing wiper and a line-testing wiper, a vacuum tube in said finder switch arranged to control the hunting operation thereof, and means for connecting the control element of said vacuum tube to the groupou -hunting operation of the finder and or shifting it into engagement with the line-testing wiper durin the line-hunting operation.

33. n a telephone-system, an automatic two-motion switch having two test wipers controlling. its two motions, respectively, a vacuum tube in said switch arranged for the control of the switch to be exercised therethrough, and means for associating the control element of said vacuum tube with the two test wipers successively.

34. Ina two-motion automatic switch, two

- relays arranged to terminate the two motions,

respectively, testing apparatus in said switch including a vacuum tube, and means for shiftingtthe plate circuit of said vacuum tube into association with said two control relays, successively.

' switches each havin the plate circuit of said vacuum tube .into

association with said two control relays, re-

spectively, a holding relay for holding the automatic switch in its attained position after the two motions have been executed, and means effective upon the completion of the secondary motion of said automatic switch for bringing said holding relay under control of the plate circuit of said vacuum tube.

36. In a telephone system, a group of sub scribers lines, a finder having access to said lines, means eflective when one of said lines is calling for operating said finder into association with said lines successively,'a vacuum tube in said finder controlling the operation thereof, means for associating the control element of said vacuum tube with said lines successively as the movement of the finder switch progresses in order to test the lmes, means controlled by said vacuum tube under the control of its control element for terminating the operation of the finder and for seizing the calling line when it IS reached, and means for maintaining the control element of the vacuum tube in connection with the calling line to maintainthe finder SWI-tCh in its operated condition until the receiver is replaced on the callingline.

37. In a telephone system, a train of serially related automatic switches for extending a talking connection from a calling line, a vacuum tube in each switch, a connection from the grid of each tube to the talking circuit, and means in the plate circuit of .each tube 'for controlling the release of the associatedv switch. p

38. In a telephone system, a train of ser ally related automatic switches fonextendmg a talking connection from a calling llne, a vacuum tube in each switch, a circuit including the calling line for supplying potential from the exchange tothe grids of said. tubes, and means in the plate circuit ofv each tube for controlling the release of the associated switch. 39. In'a telephone system, a train of sanally related automatic switches for extending a full metallic talking circuit from. a calling line, trunk lines extendin between adjacent on y two conductors, a vacuum tube in eac switch, a grid in, each tube connected to the talking circuit, and

means in the plate circuit of each tube for controlling the release of the associated switch. j I

40. In a telephone stem, a group oftrunk' lines each having a-tal ing con uctor normally maintained at a given potential,- means for taking any one of said trunk lines for uses (1 for thereupon setting up a current flow ower the said talking conductor thereof and thereby altering the potential on said conductor, an automatic switch having access to said trunk lines and having a free hunting movement so that it may effect connection with any idle one of said trunklines, and means including a vacuum tube directly responsive to the potential existing on the said conductors of said trunk lines as-the trunk lines are passed over successively by said automatic switch for causing the trunk hunting opera--- tion to continue over the trunk lines on which the potential has been altered and for causing said switch to seize the first trunk line with an unaltered v potential thereon.

41. In a telephone system, a group oftrunk lines, means efiective while any one of said trunk lines is in use for placing a characteristic potential on a conductor thereof, an automatic switch arranged to pass over said trunk lines successively and to make connection with the first one thereof not in use, and means in said switch including a vacuum tube having its grid element successively associated with said trunk lines to cause the be continued as long as the characteristic potential is encountered and to cause the first trunk line not having the characteristic potential to be taken for use. i

42. In a telephone system, lines accessible to an automatic switch, line equipment associated with each line and causing the two conductors of the line to be telephonically unbalanced, and means. including said automatic switch efiective when a connectionis made to any one of said lines to set up a condition to offset the unbalanced condition of the line and thereby set up a substantially balanced condition of the line.-

43. In a tele hone system, lines each havin individual ine eguipment inherently unba ancing the line or conversational, purposes, trunk lines or links through which talking connections may be set up from said lines to other lines, and circuit connections in said links tending to set up an equal and op posite unbalance on said lines, whereby a substantially balanced condition is obtained.

$4. In a telephone system, lines each having individual line equipment which introduces a telephonically unbalanced condition on the line, trunk lines or links, means efli'ective for connectin said 'lines to ether in pairs by way of s'ai 7 links, and a tal ing-con--' 'ductor reversal in each of-said links, wherelfiiy when a connection is to be set up through one of said lines for associating said electrostatic device with said lines successively to locate one not in use.

46. In a switching system, an automatic switch having access to lines each of which is maintained at a potential depending upon whether it is in use or not,- an electro-static device in said switch, means effective when a connection is to be set up through one of said lines for associating said electro-static device with said lines successively to locate one not in use, and means controlled by said electro-static device over a talking conductor of a connection set up over one of said lines through said switch for breaking the connection when it is no longer desired.

47. In combination, a switch for testing lines for their busy or idle condition, means for connecting said switch with said lines successively, and an electro-static testing device in said switch included in the test circuit established with each line.

48. In a telephone system, linesjcomprising two talking conductors each, an electro-static testing device, and means for connecting said device to a talking conductor of said lines successively to test said lines for a busy or idle condition.

49. In combination, a switch havin access to lines each of which is marked by a distinguishing potential when idle, means for operating said switch to make testing connections with said lines successively to determine their busy or idle condition, an electro-static device included in the testing connection made to said lines having an inherent capac- I ity so low as to preclude the disturbing of connections set up over busy lines, and means in said switch controlled by said electro-static device to indicate the busy or idle condition of the lines as they are successively tested.

50. In a switching system, lines each of which is maintained at one potential when idle and at another potential when busy, and means effective when it is desired to take a line for use for electro-staticallytesting said lines successively to determine the condition thereof without drawing current from the lines in use.

- 51. In an automatic telephone system, selective switches, means for operating a series of said switches to set up a two conductor metallic talking circuit, means for operating other of said switches in attempting connection with said talking circuit, and testing means in said other switches including vacuum tubes controlled over said talking circuit whereby such attem ted connections are barred without inter ering with conversation on said talking circuit.

52. An automatic telephone system comprising selective switches, in which the usual third conductors extending between switches and the test wipers of the switches are omitted, said system comprising means in each switch, includin a vacuum tube, whereby-the 

